1. Field of the Invention
The present invention relates generally to the field of pipe connections for sections of pipe such as those pipes used in the municipal water and sewer pipeline industries. More particularly, this invention relates to a combination sealing and restraint system for use in “as-cast” ductile iron pipeline systems.
2. Description of the Prior Art
Pipes are commonly used for the conveyance of fluids under pressure, as in city water lines. They may also be used as free-flowing conduits running partly full, as in drains and sewers. Pipes for conveying water in appreciable quantities have been made of steel, cast iron, concrete, vitrified clay, and most recently, plastic including the various polyolefins and PVC. Ductile Iron is a high strength, tough material which has traditionally been used in water and wastewater systems in all 50 states of the United States and in many other areas of the world. In the United States alone, it has been in continuous use in hundreds of municipal utility systems for more than a century.
In many applications where lengths of pipe are joined in telescoping relationship, the spigot end of one pipe is inserted into the socket end of the engaging pipe at a pipe joint or “coupling.” The socket end has an opening large enough to receive the spigot end of the mating pipe. A gasket is typically present within the socket end of the pipe which is intended to prevent leakage of fluid from the joint by forming a seal between the two pipe sections. Piping systems of the above type also typically include “fittings” as that term is defined in the industry. A fitting is a term which will be familiar to those in the relevant industries and includes a piece, often curved or at an angle, as a coupling, an elbow, a valve, a Tee, etc. used for connecting lengths of pipe or as an accessory to a pipe in a piping system for conveying fluids. Exemplary “as cast” ductile iron pipe fittings are shown, for example, in the Tyler/Union Utilities Mini-Catalogue, May 2001, on pages 2-3, as the “Mechanical Joint C153 Ductile Iron Compact Fittings.” These fittings are merely intended to be exemplary, as there are a number of other commercial sources for such pipe fittings.
In addition to pipes and fittings, there are other components in a pipeline system which are placed in fluid communication with a length of pipe, requiring a sealed coupling or joint. For example, there are hydrant tees and valves which are commonly used in most municipal water systems. As an example of such components, American AVK is a leading manufacturer of gate valves, fire hydrants, and accessories for the water, wastewater, fire protection, and irrigation industries and company catalogues illustrate a variety of these general types of products.
One important consideration in piping systems of the above type, whether in a straight run of pipe necessity of providing effective sealing, another important design requirement exists when it becomes necessary to join the pipe components in a restrained manner. This is usually desired in order to prevent the pipe components from separating due to thrust forces that often occur when the pipeline is subjected to internal or external pressures, changes in direction or elevation of the pipeline, and sometimes when earthquakes or tremors or other external factors come into play.
Thus, in a straight section of a ductile iron pipeline, the hydrostatic forces are generally balanced. Wherever the pipeline changes direction or diameter, such as at a bend, tee or a reducer, however, the hydrostatic forces create an unbalanced thrust force in the line. This unbalanced thrust force can cause the line to move or its joints to separate unless the thrust force is counterbalanced in some manner. Where there are only gradual changes of direction in the line, the lateral thrust forces are normally counterbalanced by the friction between the pipe and the soil along the length of piping, and joint restraint is not normally required. However, when higher pressures, poor soil conditions or significant changes of direction or diameter are encountered, the thrust forces may be too great to be resisted by the soil surrounding an unrestrained joint. In the past, these unbalanced thrust forces have commonly been counterbalanced with thrust blocks, restrained pipe joints, or a combination of the two.
Conventional ductile iron pipe joints have been provided of both the “push-on” type joints, as well as the traditional bolted mechanical joints. For example, one of the most common devices for forming a sealed joint in straight runs of pipe referred to above is the “mechanical joint” or “MJ”. There, the bell end of an iron pipe section has a cast-on flanged portion on the pipe exterior. The spigot end of a second iron pipe is fitted with a slidable gland fitting and a gasket that is conically shaped such that one face of the gasket is diametrically larger than the second face of the gasket. The conically shaped gasket is positioned between the gland fitting and the spigot end of the pipe with the smaller, second face of the gasket being closer to the spigot end than the larger, first face of the gasket. The gland fitting has a plurality of apertures for receiving standard bolts. The joint is formed when the spigot is axially inserted into the bell, and the gland fitting and the flanged portion are pieces.
While the “internal” gasket used in the traditional MJ design for ductile iron pipe served to seal the joint, the gasket did not feature a cooperating “restraint” feature in order to assure the greater integrity of the joint of pipe. Instead, it was necessary to utilize a cumbersome external mechanical restraint system made up of the flange, bolts, screws, etc., as discussed above. Also, when the pipe component being joined was a fitting rather than a straight run of pipe, there was less room available on the exterior of the fitting to accept the various parts (flanges, bolts, screws, etc.) that were necessary to make up the MJ type restraint.
In order to meet the need for a restrained joint of the above type, a number of companies in the industry have worked to develop various forms of restrained joint products for ductile iron pipelines. These include, for example, the FAST-GRIP® and TR FLEX® restrained push-on joints, the FIELD LOK 350® gaskets used to restrain push-on TYTON®JOINTS, and the more recent MJ FIELD LOK® gaskets which are used to restrain mechanical joint pipe and fittings.
The Ductile Iron Pipe Research Association (DIPRA) has published the “Thrust Restraint Design for Ductile Iron Pipe,” a document of conservative design guidelines for the restraint of thrust forces in underground, pressurized, Ductile Iron piping systems. The DIPRA procedures are based on accepted principles of soil mechanics and provide formulas for determining thrust forces and the necessary restraint.
Thus, in spite of improvements in pipeline systems generally, a need continues to exist for an improved sealing and restraint system, particularly for ductile iron pipelines, of the type which offers complimentary sealing and self-restraining features.
A need also exists for such a system which is cost-effective, easy to manufacture and easy to use in the field and which is dependable in operation.
A need also exists for such a system which effectively restrains ductile iron pipes and fittings, as well as hydrant tees and valves, against internal and external forces without the need for an external flange, bolts or associated restraining screw mechanisms.
A need exists, accordingly, for a sealing and restraint system for ductile iron pipelines, and particularly for fittings, which offers complimentary sealing and self restraining features in an internal sealing/restraint system.